Fitness profiling of the S. mutans genome. Caries formation by S. mutans requires the organism to form a biofilm, become a prominent member of the microbial community of dental plaque and to survive stresses within plaque (Marsh, 2003). The initiation of caries involves demineralization of tooth enamel by organic acids produced by S. mutans fermentation of sugars. S. mutans survives acid environments by the up- regulation of a number of proteins, or, fatty acids in a process referred to as acid-adaptation. Evidence from our laboratory, and others, shows that acid-adaptation is a complex, multi-gene process that is only partially understood at present. With the availability of the S.mutans genome sequence, the annotation of 1,963 open reading frames, and the development of methods precisely to delete individual open reading frames, it is possible to analyze every nonessential gene in S. mutans. In this project, we will examine the effect of each gene on the ability of S. mutans to grow in acidic conditions. The contribution of each gene in the organism to events implicated in caries formation will be determined. Specific Aim 1a. High throughput methods will be tailored to reproducibly delete individual genes in the entire S. mutans genome. Each deletion will be marked by the presence of two unique DMA sequences (18-20 bp) such that abundance of each particular mutant strain in a population of mutants and wild type can be scored using these "bar codes." Specific Aim 1b. A genomic set of marked deletions will be constructed, resulting in a library of S. mutans strains bearing knockouts of all non-essential genes. Methods to analyze gene function by fitness profiling will be used to determine a function for each gene in S. mutans. Specific Aim 2. To find genes that may affect growth in the oral cavity, we will test mutant strains for their ability to grow, in vitro, in acidic conditions. Then, we will use the knockout library in fitness profiling experiments under defined conditions, which will include the following: 1) co-culture of the library of mutant strains growing in acidic environments, including planktonic and biofilm cultures;and, 2) a mouse infection model, in which the mice are fed a highly cariogenic diet. These experiments will be done as screens for candidate strains to be examined individually in the mouse caries model. Specific Aim 3. We will also examine the ability of the library of mutant strains to grow in the presence of additional species of oral microbes, with the aim of identifying those genes in S. mutans that contribute to its ability to out-compete other oral organisms. From these studies, we expect to identify genes in S. mutans that participate in acid-resistance. The relevance to public health is that the information derived from these studies will lead to new insights, and potentially new targets for therapeutic intervention, into specific mechanisms that S. mutans uses to cause disease in humans.